Method for producing porous metal battery electrode structure

ABSTRACT

A method for the preparation of highly porous metallic battery electrode structures by the action of pressure alone without the application of heat is described in which the surface of the powders used for the compact is cleaned by the action of mild acids prior and during the pressing operation. In a first modification, a mixture of metallic powder, a powdered nonreactive pore-forming agent and a mild acid are pressed to form a porous structure. In a second modification, a mixture of metallic powder and a powdered slightly soluble mild acid are pressed to form a porous structure. In the two modifications the pore former is removed after the pressing operation.

Hein et al.

[ 51 Feb. 29, 1972 '[22] Filed:

[72] Inventors: Edward R. Hein, Doylestown; William E. Veit,1vyland,both of Pa.

ESB Incorporated June 1, 1970.

[21] Appl. No.: 411,883

[73] Assignee:

[52] U.S. Cl ..136/24, 136/30, 136/126, 264/111 [51] Int. Cl. ..1101m13/08 [58] Field oiSearch ..l36/24,21,30,29,76, 120, 136/125-126,130,120, 95, 102; 264/111; 75/211, 214, 222; 29/192 [56] ReferencesCited UNITED STATES PATENTS 2,501,673 3/1950 Glassner ..136/1303,322,535 5/1967 Rao ...75/20l 3,337,336 8/1967 Rao ..75/201 3,348,97610/1967 Kelly et al ..l36/125 3,378,369 4/1968 Smith ...75/206 3,380,8224/1968 Kelly et ..75/2l4 3,384,482 5/1968 Kelly et a1 ..75/201 3,413,11611/1968 Rao ..75/212 3,418,113 12/1968 Rao ..75/214 PrimaryExaminer-Anthony Skapars Attorney-Alfred .1. Snyder, Jr., Robert H.Robinson, Raymond L. Balfour and Anthony J. Rossi ABSTRACT A method forthe preparation of highly porous metallic battery electrode structuresby the action of pressure alone without the application of heat isdescribed in which the surface of the powders used for the compact iscleaned by the action of mild acids prior and during the pressingoperation. 1n a first modification, a mixture of metallic powder, apowdered nonreactive pore-forming agent and a mild acid are pressed toform a porous structure. In a second modification, a mixture of metallicpowder and a powdered slightly soluble mild acid are pressed to form aporous structure. in the two modifications the pore former is removedafter the pressing operation.

7 Claims, 1 Drawing Figure PAIENTEI} FEB 2 9 1922 w" E w F I O 3 8 2 S OO5H/5H SA asvnoA BGONV INVENTORS.

EDWARD R. HEIN WILLIAM E. VEIT METHOD FOR PRODUCING POROUS METAL BATTERYELECTRODE STRUCTURE BACKGROUND OF THE INVENTION 1 Field of the InventionThis invention is directed to electrode structures for use in batterycells. In particular, it describes porous electrodes of the consummableand nonconsummable type for use in primary and secondary cell types.

2. Description of the Prior Art Porous metallic structures are usedextensively as electrodes in many forms of primary and secondaryelectrochemical batteries.

Experimenters have found that zinc and cadmium powders, although verydesirable as electrode materials, cannot be converted by the ordinarymetal powder fabrication techniques of pressing and sintering to porousstructures having a high porosity. For example, metallic zinc powderscannot be converted to a unified highly porous structure by the singleor joint action of heat and pressure. Nor has it been possible toproduce the desirable'porous structures by the action of heat andpressure in the presence of a reducing gas such as hydrogen. The sameremarks are true of cadmium powders. Compacts can be made by pressingtechniques having porosities up to say percent. When compacts are madeat lower pressures to give greater porosity, they have little or nostrength and are not suitable for battery electrode purposes.

Metallic cadmium and zinc powders form a minute surface coating of oxideupon exposure to air. This thin layer serves to protect the metal fromfurther oxidation. However, it also prevents the metal, for example, inpowder form, from adhering to itself.

If the powders are compacted at a sufiicient pressure, the oxide coatingwill be broken to form a structure. However, this pressure is so greatthat the particles are distorted and the structure has a low porosity.

The oxide layer can be removed by the chemical reaction between hydrogenand oxygen at a suitable temperature. Unfortunately, the temperature atwhich this reaction takes place is, in the case of zinc or cadmium, wellabove the melting point of the metal. Because of this, the reaction withhydrogen cannot be used to form porous zinc or cadmium structures.

Accordingly, since ordinary powder fabrication techniques cannot beresorted to to produce the porous metallic electrode structures of thetype here contemplated, other methods of preparation have beendeveloped.

A common method for preparing porous zinc electrodes involves making apaste of zinc oxide with or without binding agent, smearing this mixonto a metallic conductive grid, drying the plate and then converting itto zinc by electrolysis. In general, it has been found that it isdifiicult to make thick structures (on the order of one-twentieth of aninch thick or thicker) by this process.

In another method which has been proposed for the formation of porouszinc structures, zinc powder is mixed with a porefonning powder whichconsists of an ammonium halide salt. The halide salt is removed bysublimation at a chosen elevated temperature and under vacuum.

At the temperature chosen, the ammonium halide salt reacts with theoxide layer in a dry reaction cleaning the surface of the metal. Thetemperature is sufficiently high to permit sintering of the metalparticles and results in a desirable, highly porous, mechanically strongstructure. This method sutfers from the complications of vacuumsublimation and the attendant problems of collection and control of avaporized halide salt as well as a sintering operation at reducedpressure.

SUMMARY OF THE INVENTION In accordance with this invention, zinc orcadmium metal powder is mixed with a water solution of an ozidedissolving acid and pressed to give a strong porous metallic electrodestructure.

In a first modification, a nonreactive pore-forming agent such asnapthalene is incorporated in the mix.

In a second modification, a slightly soluble acid such as benzoic acidis used as combined cleaning agent and pore former.

In a third modification, a mercury salt of the cleaning or other acid isadded to the solution to give an evenly divided mercury coating to theporous metal structure.

DISCUSSION OF THE INVENTION In the first embodiment of this invention,zinc or cadmium metal powder of suitable size is mixed with a sufficientquantity of an acid which will rapidly dissolve the metal oxide butwhich reacts only slowly with the base metal, such as acetic, benzoic,tartaric or weak hydrochloric acid to wet all particles and form aslurry. The slurry is spread out in a pressing die and pressed at apressure between about 800 and 20,000 pounds per square inch to form aporous metal structure. It is found that compacts pressed at highpressure will have all the aqueous solution removed by the compaction ofthe powder. However, compacts not pressed as hard will contain someresidual solution. These latter compacts are therefore promptly washedin water and dried. To prepare an electrode, a conductor is attached.Alternately, a conductor can be placed in the pressing die prior to thepressing operation. The porosity of the finished plate will bedetermined principally by the mixture of powder size range, the powderparticle shape and the pressing pressure as is known to the art. Thegreater the compacting pressure, the lower the porosity and the strongerthe electrode. Porosities from near zero to about 45 percent can beproduced by this method.

In the second embodiment and in order to get a higher degree ofporosity, a pore forming agent is dry mixed with the metallic powderprior to the addition of the oxide removing solution. The pore formingagent permits the use of a highcompacting pressure with consequentstrong bonding of the metal powder particles. The pore-forming agentmust be insoluble and unreactive in the cleaning acid, it should have auniform particle size and it must be readily removable from the finishedelectrode. Further, it must be capable of flow under pressure and musttend to bond with itself under pressure to form tendrils which willbecome connecting pores when it is removed from the structures. Finally,it must not coat the metal particles or otherwise prevent metal-to-metalcontacts. A suitable material for this purpose is napthalene, althoughother functionally equivalent materials will be suggested to one skilledin the art.

To complete the electrode, the dry mix or pore former and metal powderis treated with a solution of the oxide dissolving acid. The resultingslurry is then spread out in the pressing die, pressed and washed. Thenapthalene can be washed out of the finished electrode by the use ofsolvent such as benzene. It also can be removed by evaporation at atemperature of about 70 C. and a reduced pressure of about 5 mm. mercuryabsolute. Electrodes of satisfactory strength and having porosities upto perhaps 75 percent have been produced by this means.

In order to simplify the processing, a pore former can be chosen whichis an acid and is only slightly soluble. Benzoic acid is typical of thisclass. A dry mix of metal powder and benzoic acid is made. It is thenwet with water, placed in a suitable die and pressed. After pressing,the benzoic acid can be removed by washing with hot water or by solvent.

It is well known in the art that for some services, it is highlydesirable to coat the surface of zinc electrodes with mercury. In thecase of this invention, mercury can be chemically coated on the zincsurface by including, for example, a mercury salt of the cleaning acidin the acid solution. The mercury will be precipitated from the solutionby chemical displacement. It is not actually necessary to use the saltof the cleaning acid and in the case of the slightly soluble acid poreformers, the mercury salts may not be sufiiciently soluble. Therefore,in this embodiment of the invention, other mercury salts are desirable.

This method of mercury addition has the advantage over other methods ofmercury addition of giving a very uniform apportioning of the mercuryover the entire active surface of the electrode. The calculations fordetermining the correct amount of mercury salt to supply a given amountof mercury to the plate are well known and therefore are not included inthis specification.

It is normally not necessary to add mercury to porous cadmiumelectrodes.

Some typical examples of several of the embodiments of the invention andan example of the behavior of a typical battery electrode prepared inaccordance with the invention follow.

EXAMPLE 1 20 grams of 40 mesh zinc powder was mixed with 2.5 ml. of 20percent acetic acid. The mixture was pressed in a die at 800 p.s.i. Theresulting compact had a porosity of 44 percent. It had good structuralstrength and could be readily handled.

EXAMPLE 2 20 grams of 40 mesh zinc powder was mixed with 2.5 ml. of 20percent acetic acid. The mixture was pressed in a die at 4,000 p.s.i.The resulting compact had a porosity of 41.5 percent. It was strong andcould be readily handled.

EXAMPLE 3 5.5 grams of plus 40 mesh cadmium filings was mixed with 2.7ml. of 20 percent acetic acid. The mixture was pressed in a die at 4,875p.s.i. The resulting compact had a porosity of 30.6 percent. It had goodstructural strength and could be readily handled.

EXAMPLE 4 5.5 grams of plus 40 mesh cadmium filings was mixed with 2.2ml. of 20 percent acetic acid. The mixture was pressed in a die at 3,250p.s.i. The resulting compact had a porosity of.37.6 percent. It wasstrong and could be readily handled.

EXAMPLE 5 5 grams of 50+200 mesh zinc powder was mixed with 0.1 gms. ofpowdered tartaric acid and sufficient water to make a paste. The pastewas placed in a die and pressed at 4,875 p.s.i. The resulting compacthad a porosity of 37.8 percent. It had good structural strength andcould be readily handled.

EXAMPLE 6 A 5 grams of 325 mesh zinc powder was mixed with 0.1 grams ofpowdered benzoic acid. Sufficient water was added to make a paste. Thepaste was placed in a die and pressed at 4,875 p.s.i. The resultingcompact had a porosity of 33.5 percent. lt was strong and could behandled.

EXAMPLE 7 5 grams of 325 mesh zinc powder was mixed with 1.5 gms. ofpowdered benzoic acid. Sufiicient water was added to make a paste. Thepaste was placed in a die and pressed at 4,875 p.s.i. The remainingbenzoic acid was leached out of the compact in hot water. The resultingcompact had a porosity of 67 percent. Some loss in strength was notedbut the compact could still be handled.

EXAMPLE 8 10 grams of 50+200 mesh zinc powder was mixed with 2 ml. of asolution of percent acetic acid containing 200 g. per liter of mercuricacetate. The mixture was placed in a die and pressed at 4,000 p.s.i. Theresulting compact was completely amalgamated and had a porosity of 37.6percent. It had good structural strength and could be readily handled.

EXAMPLE 9 24 grams of -40+200 mesh zinc powder was mixed with 9 g. offlake naphthalene in ajar mill for l hour. 9.5 ml. of 20 percent aceticacid containing 200 g. per liter of mercuric acetate was then added. Themixture was placed in a die and pressed at 20,000 p.s.i. The resultingcompact was removed from the die and heated at 70 C. under a vacuum of27 inches of mercury for 3 hours. This removed all traces ofnaphthalene. The resulting structure was strong. It had a porosity of 70percent.

EXAMPLE 10 22 grams of 40+200 mesh zinc powder was mixed with 7 grams offlake naphthalene in a jar mill for one hour. 8 ml. of 20 percent aceticacid containing 200 g. per liter of mercuric acetate was then added. Themixture was placed in a die and pressed at 20,000 p.s.i. The resultingcompact was removed from the die and the naphthalene was washed out byrefluxing in trichlorethane. The porous compact formed in this way wasstrong and had a porosity of 65 percent.

EXAMPLE 1 l 220 grams of 40-1-200 mesh zinc powder was mixed with 70 g.of flake naphthalene in a jar mill for 1 hour. ml. of 20 percent aceticacid containing 200 g. per liter was then added to the dry mix. The wetmixture was placed in a die and pressed at 4,500 p.s.i. Some liquid wasforced from the die. There was no readily detectable mercury in thisliquid. The compact was then removed from the die and heated at 70 C.under a vacuum of 27 inches of mercury for 3 hours. This removed alltraces of naphthalene. The resulting structure was strong and had aporosity of 66.5 percent.

EXAMPLE 12 15.4 grams of -40+20O mesh zinc powder was mixed with 2.5 ml.of 2.5 N HCl. The wet mixture was placed in a die and pressed at 4,875p.s.i. The compact had a porosity of 37 percent and had good structuralstrength.

EXAMPLE l3 1 1 grams of 40+200 mesh zinc powder was mixed with 3.7 g. ofnaphthalene in a ball mill for 1 hour. To this dry mix 3 ml. of 2.5 NHCl was added. The wet mix was placed in a die and pressed at 16,000p.s.i. The naphthalene was removed from the compact by heating at 70 C.under a vacuum of 27 inches of mercury for 3 hours. The resultingstructure was strong and had a porosity of 63 percent.

EXAMPLE 14 As an example of the use of these electrodes, an electrodewas fabricated in accordance with Example 9. The electrode was 2.5inches by 1.5 inches by 0.20 inch thick. Its porosity was 70 percent.its weight 24.95 g. and its theoretical capacity 20.2 AH. It wasdischarged at 1.25 amperes against an auxiliary electrode in 27 percentKOH. A total of l6.9 ampere hours was withdrawn from the electrode withan ampere hour efficiency of 87.9 percent. The discharge voltagetimecurve is shown in the accompanying drawing. The slight rise in voltageduring the discharge is due to the effect of a rising concentration ofpotassium zincate in the electrolyte on the mercurymercuric oxidereference electrode.

Having fully described our invention and given examples of itsembodiments and its utility, we claim:

1. A method for preparing porous metal structures for battery electrodesfrom a material that is not readily sinterable wherein the material iscadmium or zinc which comprises:

a. mixing the metal in powder form with an aqueous solution of an acidselected from the group which consists of acetic acid and hydrochloricacid to produce a slurry;

b. placing the slurry in a form; and

c. pressing the slurry in the form at a pressure between 800 and 20,000p.s.i.

2. A method as defined in claim 1 in which the metal is zinc and amercury salt of the acid is mixed with the acid solution.

3. A method for preparing porous metal structures for battery electrodesfrom a material that is not readily sinterable wherein the material iscadmium or zinc which comprises:

a. mixing the metal in powder form with a powdered pore forming agentwith the characteristics of insolubility, unreactiveness in watersolution, and capable of flow and self-bonding under pressure to form adry mix;

b. adding to the dry mix an' aqueous solution of an acid selected fromthe group which consists of acetic acid and hydrochloric acid to form aslurry;

c. placing the slurry in a form;

d. pressing the slurry in the form at a pressure between 800 and 20,000p.s.i.; and

e. removing the pore-forming agent.

4. A method as defined in claim 3 in which the metal powder is zinc anda mercury salt of the acid is mixed with the acid solution.

5. A method as defined in claim 3 in which the pore former is powderednapthalene.

6. A method for preparing porous metal structures for battery electrodesfrom a material that is not readily sinterable wherein the material iscadmium or zinc which comprises:

a. mixing the metal in powder form with a dry slightly soluble organicacid selected from the group consisting of benzoic acid and tartaricacid;

b. adding water to the dry mix to form a slurry;

c. placing the slurry in a form;

d. pressing the slurry at a pressure between 800 and 20,000

p.s.i.; and

e. removing the residual organic acid.

7. A method as defined in claim 5 in which the metal powder is zinc anda mercury salt is added to the components of the slurry.

* i i i

2. A method as defined in claim 1 in which the metal is zinc and a mercury salt of the acid is mixed with the acid solution.
 3. A method for preparing porous metal structures for battery electrodes from a material that is not readily sinterable wherein the material is cadmium or zinc which comprises: a. mixing the metal in powder form with a powdered pore forming agent with the characteristics of insolubility, unreactiveness in water solution, and capable of flow and self-bonding under pressure to form a dry mix; b. adding to the dry mix an aqueous solution of an acid selected from the group which consists of acetic acid and hydrochloric acid to form a slurry; c. placing the slurry in a form; d. pressing the slurry in the form at a pressure between 800 and 20,000 p.s.i.; and e. removing the pore-forming agent.
 4. A method as defined in claim 3 in which the metal powder is zinc and a mercury salt of the acid is mixed with the acid solution.
 5. A method as defined in claim 3 in which the pore former is powdered napthalene.
 6. A method for preparing porous metal structures for battery electrodes from a material that is not readily sinterable wherein the material is cadmium or zinc which comprises: a. mixing the metal in powder form with a dry slightly soluble organic acid selected from the group consisting of benzoic acid and tartaric acid; b. adding water to the dry mix to form a slurry; c. placing the slurry in a form; d. pressing the slurry at a pressure between 800 and 20,000 p.s.i.; and e. removing the residual organic acid.
 7. A method as defined in claim 5 in which the metal powder is zinc and a mercury salt is added to the components of the slurry. 